With increasing frequency insulating layers are applied to outside walls, and such layers are subsequently spackled or plastered. The layers are formed from, e.g., sheets, plates or blocks (hereafter: slabs), generally from 20 to 30 millimeters (mm) in thickness, of a plastic material, such as polystyrene hard foam or polyurethane hard foam.
Because of the very high thermal-expansion coefficient of polyurethane hard foam and the attendant movements brought about by temperature changes, plaster coatings are frequently cracked or torn open at points or areas of contact between the plaster and such hard-foam insulation. When, e.g., polystyrene hard-foam slabs are used, as is the case in many buildings, cracks readily form in the plaster coating over the contact surface and particularly at the juncture between insulating slabs, especially at those places where thicker insulating layers or fresh polystyrene hard-foam slabs are used.
Due to the increasing cost of heat energy, insulating-material thickness of at least about 30 mm are required. Today, an insulating-material thickness of about 70 mm is generally considered to be optimum. For electrically-heated buildings, the optimum figure may even be as high as 180 mm. For such thick insulating layers polystyrene hard-foam slabs, which are smooth on both sides, are unsuitable because forces developed at the interface between the slabs and plaster coating thereon become so great that they exceed the physical limits of the plaster. The insulating slabs are subjected to inherent movement due to shrinkage, as well as to expansion and contraction with increasing and decreasing temperatures. The leads to excessive stress on the plaster coating, particularly that which is over joints between slabs.
The use of grooved hard-foam slabs provides a larger contact surface and results in stronger adherence between the slabs and mortar applied thereto. The adherence is enhanced by the mortar which enters the grooves and thus forms a further interlock with the slabs. Unfortunately, the adverse effects of shrinkage are retained. Such shrinkage is reduced by storing the slabs for an extended period of time prior to use. Such extended storage tends to minimize residual follow-up shrinkage. When the preliminary storage time is insufficiently-long, damage from such shrinkage cannot be avoided and is merely postponed. Storage times of about six months are now customary.